The present invention relates to a method and apparatus for monitoring an optical network being equipped with WDM equipment.
WDM means xe2x80x9cWavelength Division Multiplexingxe2x80x9d and is a technique known to those skilled in the art and will therefore not be described in detail.
Basic monitoring parameters in an optical network are:
Total optical power
Number of data channels
Noise on every channel
These monitoring parameters are processed to give the following alarm signals:
Loss of optical signal (LOS)
Signal per Channel Degradation
Loss of channel
Generally, optical networks are being equipped with WDM equipment to enhance the data capacity of the optical transmission lines. A crucial point in the management of an optical WDM network is optical monitoring of the network channels. A WDM optical network layer transmits a plurality of data channels at different optical wavelengths over an optical fibre. To manage such a network layer the optical network layer is to be monitored at certain points in the network to obtain data on the quality of each channel. Failure or degradation of one of the elements in the optical network could cause the loss or degradation of one or more data channels, respectively, at a node in the optical network layer. It is up to the monitoring system of the network to report failure or degradation of channels. Existing monitor methods include pilot tones, optical bandpass filtering, photodiode arrays, waveguide gratings, and the use of optical wavelength spectrum analysers. The disadvantage of these monitor solutions is the high cost of the used equipment to realize optical monitoring.
One of the cheapest ways to monitor an optical signal is its direct detection on a photodiode, One of the major disadvantages of direct detection is the limited bandwidth of the photodiode which cannot detect optical frequencies, but only the modulated data of a data channel in the electrical domain. If an optical signal, containing multiple data channels at different optical wavelengths, is received an the photodiode, the modulated data of all data channels is transposed to the electrical domain. In the frequency and time domain of the photodiode current, the data channels become therefore unidentifiable.
FR-A-2740282 discloses an optical power measuring circuit for measuring optical power distributions of an optical signal containing multiple data channels at different optical wavelengths. The power measuring circuit includes a optoelectrical converter preceded by a tuneable wavelength filter. In the power measuring circuit the optical signal is filtered by means of a tuneable wavelength filter successively for each channel wavelength, and after conversion to the electrical domain the optical power is measured separately for each filtered signal. The measured power values are used for power control of the various optical signal sources. This known technique has among others the following drawbacks. The use of a tunable wavelength filter which should be tuned successively to the various wavelength channels, is rather expensive and does inherently not allow measuring the state of the various wavelength channels simultaneously.
In view of the above disadvantages there is a strong need for a monitor system for optical networks which can meet the following demands:
Low cost
Monitoring of multiple data channels simultaneously
Small amount of equipment necessary to realize the monitoring
Local monitoring
Independent on the location in the network
Independent on the configuration of the network
Independent on the number of data channels to be monitored.
The present invention now fulfils this need and the invention therefore provides a method for monitoring an optical network being equipped with WDM equipment able to transmit a plurality of data channels at different wavelengths, according to the preamble of claim 1, for the definition of which the above cited document FR-A-2740282 was applied, which method is, according to the invention, characterised by the characterizing part of claim 1.
The invention further provides an apparatus for monitoring an optical network being equipped with WDM equipment able to transmit a plurality of data channels at different wavelengths, according to the preamble of claim 10, for the definition of which also the above cited document FR-A-2740282 was applied, which apparatus is, according to the invention, characterised by the characterising part of claim 10.
It is remarked that DE-A-195 04 896 discloses a method and apparatus for monitoring an optical network, wherein one data channel is monitored at the same time.
However, the specific statistical analysis of an optical signal comprising a plurality of data channels and the simultaneous monitoring of a plurality of data channels according to the present invention has not been disclosed and cannot be derived therefrom.
The present invention has been based upon the fact that all data channels are statistically independent on each other. Therefore, the data channels should be separable by using a statistical analysis, The common parameters used in the said statistical analysis are the average power levels at the xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d bits and the noise on these levels for each data channel. So carrying out a statistical analysis of the optical signal containing multiple data channels should provide the noise and average power levels of each data channel and the number of channels present.